1. Field of the Invention
This invention relates generally to a temperature sensor, and more particularly to a semiconductor temperature sensor which employs the difference in base-emitter voltages of matched transistors to provide an output which is directly proportional to absolute temperature.
2. Prior Art
In a copending application for U.S. Patent of Robert C. Dobkin, entitled TEMPERATURE TRANSDUCER, Ser. No. 477,323, filed June 7, 1974, now U.S. Pat. No. 4,004,462, and assigned to the same assignee as the present invention a temperature sensor is disclosed which employs the difference between the base-emitter voltages of at least two transistors to provide an output which is directly proportional to temperature in a known temperature scale. That application is referred to for a discussion of the problems associated with presently known temperature sensors, and transducers employing such sensors.
One of those problems relates to the need for a transducer which does not require wiring to remote locations for being supplied with regulated voltage or for amplifying, scaling, and performing other operations on its output before such an output can be utilized for measurement or control. As also discussed in that application, a need exists for a temperature sensor and transducer in which power dissipation is relatively low. The majority of the presently available temperature sensors can not be easily implemented in measurement and control systems. In addition, thermocouples have relatively low output signals which are difficult to amplify with any degree of stability. In addition, thermocouples require cold junction compensation. Resistance and thermistor sensors are nonlinear and excitation dependent. Another disadvantage of such sensors, however, is that their outputs are not directly related to any known temperature scale.
The temperature sensor discussed in the above-mentioned copending application employs the difference between the base-emitter voltages of transistors to generate an output which is proportional to temperature. Such an output can only be achieved if the ratio of the current density of one transistor to the current density of the other transistor is maintained constant with changes in temperature. A relatively large number of components, particularly matched transistors, are required to maintain the current density ratio constant with temperature changes. The sensor disclosed in that application is also not easily calibrated. Furthermore, that sensor circuit requires relatively high current gain to obtain the desired results.
In addition to the above mentioned disadvantages of prior known temperature sensors, a need exists for a two-terminal, self-contained temperature sensor in which only two wires are required to provide power to the sensor and to provide an indication to a remote location of the sensed temperature. Additionally, a need exists for a temperature which is easily adjusted or calibrated to provide an output which is linear and directly related to a known temperature scale.